The present invention relates to a image processing device and more particularly to a gray level image processor which is capable of improving the quality of print of a gray level image by processing the quantized image data by performing operations on interpixel spacing and which includes a pseudo gray level image processing device capable of varying thresholds according to original images and environmental conditions and which is intended for use in, e.g., digital copying machines, facsimiles, printers and so on.
In the case of the conventional processing of gray level images, quantized image data is directly printed out. For instance, image data input in digital form contains information of the pixels, each having a gray level in the range of, e.g., 0 to 255. This means that a stream of 8 bits is required to represent each pixel of gray level in the range of 0 to 255 if no additional processing is made. A tremendous memory capacity may be required to store whole image data. In addition, there must be a printer which is capable of reproducing images of gray levels varying from 0 to 255. In order to reproduce scanned images of such gray levels by printers having a low memory capacity and a relatively low power for reproducing the gray shades of an image, it is necessary to reduce the amount of information to be carried by a pixel and to decrease the number of gray shades in each pixel.
The processing procedure begins with reading image data containing pixels having gray levels of 0 to 255. The read-in data is quantized for the gray levels of 0 to 255.
For instance, the gray levels (0 to 255) of the data are quantized at points W, X, Y and Z. The quantization is made to discriminate the input data "f" with reference to preset thresholds t1, t2 and t3 as follows: EQU W if 255.gtoreq.f&gt;t1; EQU X if t1.gtoreq.f&gt;t2; EQU Y if t2.gtoreq.f&gt;t3; EQU Z if t3.gtoreq.f.gtoreq.t0.
However, the quantized image data only may not faithfully represent the gray levels of the local areas in an original and may lack the gradual passing of one shade to another within an image. To eliminate the above-mentioned defects, differences of tonal densities between the image and the original, which occurred in the quantization process, is detected as an error that is processed to give the effect of tonal densities in pixels around an attentional one. Thus processed, quantized image data can more convincingly represent the gray levels in the details of an original image.
An original image is optically scanned by exposing the system of an original image scanner and the reflected light from the original image creates an image on a charge coupled device (CCD) by which it is converted into analog voltage signals corresponding to the tonal densities of pixels of that image. These analog signals are converted by an A-D converter into digital signals that are corrected for various parameters and then output as digital signals each of which, e.g., 8 bits representing tonal densities (up to 256 shades) of corresponding pixels. These digital signals are used for turning a laser output portion ON and OFF. It converts the digital signals into light image signals again to be recorded by an image recorder. However, the reproduction of 256-gradational representations of each pixel of the image requires a very large amount of storage, which makes it of no practical use. Therefore, the 256-gradational (8 bits per pixel) quantized information is converted into, e.g., 4-gradational (2 bits per pixel) quantized information that may be simply restricted by specific thresholds.
As described above, the conventional device has no means to selectively use a plurality of processing modes according to the kinds of original images to be processed and therefore may give only a fixed gradational representation using fixed quantized values and thresholds. In short, since a processed image may have a fixed gray level representation independent of other kinds of original images, it is necessary to adjust the gray levels of the image data by varying the ON-OFF duration of its exposure to laser light according to preset laser values.
The above-mentioned prior art will be further described in detail as follows:
Input data is read by the image reading unit as digital information representing pixels each having gray levels of up to 256. Quantizing values and thresholds for converting high gradation data (256 gray levels per pixel) into low gradation data is then determined. The data on the gray levels of 0 to 255 are divided into 4 quantized values A, B, C and D (representative of the gray levels) and a threshold between the quantized values A and B is defined as t1. Similarly, the threshold between the quantized values B and C and the threshold between C and D are defined as t2 and t3 respectively. Laser (images) gradation is set to represent half-tone image by changing the binary surface area.
As described above, the prior art has such drawbacks that it performs quantization of gray level image data by using fixed quantizing values and fixed thresholds and, thereby, requires changing laser ON time per pixel representing laser gradation to change the surface gray level of an image printable by an image recorder; and it is hardly adaptable to possible changes in environmental conditions such as changes in the luminosity of an exposure lamp with the time of use.
The Japanese publication of examined application JP 61-29502 discloses an image stabilizing device that previously measured gray levels of toner images of a dark portion (without being exposed) and a light portion (exposed to light of a specified intensity) produced on a light-sensitive material by an optical densitometer and performs the control of the electrophotographic(xerographic) process to create an electrostatic latent image by regulating the charged voltage put out on the basis of a dark portion toner image and by regulating the exposure conditions or a bias voltage for toner image development.
The Japanese publication of unexamined application JP 1-97065 discloses a facsimile device which includes a plurality of half-tone image processing circuits having different characteristics and which selectively uses these circuits each for the most suitable purpose of achieving a high grade of performance.
As mentioned above, the conventional gray level image processing method allows to directly print only quantized data of a gray level image whose gradation is represented by dot size of a dot-matrix of the printer portion. Consequently, the quantized dotted image representation may be random and contain separated, closed or joined dots therein when the dots are laid in different positions due to a variation in printer driving pulses. In other words, it may be easily effected by other factors than image processing.
There is shown by dotted lines a relationship between gray level values of 0 to 255 and tonal densities of a copy image in the case of a conventional method. The problem exists in that the copy image may vary absurdly in tonal density and thereby cannot smoothly represent a gray level image. In addition, there is still another problem, that a conventional circuit for gray level image processing does not include the discrimination of the edge and non edge areas of an image, thereby separately correcting only the gray level.
As mentioned above, the conventional gray level image processing method performs the quantization of gray level image data using fixed quantizing values and fixed thresholds and, therefore requires complicated settings of laser side gradation to adjust the gray levels of the image to be reproduced. It is difficult to attain optimal gray levels according to those kinds of originals. It is also difficult to be optimally correct for the possible deviation of the gray levels of an image due to changes in the environmental conditions, e.g., a change in the luminosity of an exposure lamp with the time of use.
The image stabilizing device disclosed in the Japanese publication of examined application JP 61-29502 controls the electrophotographic(xerographic) process according to signals of a light portion and dark portion of a toner image and therefore can perform adjustments of gray level only at one set point. However, it is incapable of separately adjusting two or more gray levels and of producing a high quality copy of an original image through the fine adjustment of its reproduce ability.